This invention relates to an optical waveguide for the middle infrared band.
The CO.sub.2 gas laser which emits light in the middle infrared band region in the neighborhood of 10 .mu.m is widely used for laser machining because of high efficiency and relatively low cost. The main body of this laser has a fairly large size of at least 1 meter and requires use of high voltage and cooling water. It has been customary, therefore, for the laser beam to be transmitted from the main body of the laser to the point of laser machining by means of a mirror waveguide system. The mirror waveguide system for the transmission of light is complicated in construction and difficult to handle. Increasing interest, therefore, is being attracted by the optical fiber which is easy to handle. In the adoption of the optical fiber for the transmission of CO.sub.2 laser light, it is only natural that the core material of the optical fiber should cause as low loss of light at the aforementioned wavelength as permissible. For the wavelength of 10.6 .mu.m, for example, alkali halides (such as NaCl and KBr), silver halides (such as AgBr), and thallium halide (such as KRS- 5) may be cited as advantageous materials for the core. These core materials, however, are not easily vitrified and they are deficient in fabricability. Besides, they permit no ready formation of the two-layer construction of core and clad, lack mechanical, thermal, and chemical stability, yield readily to entry of impurities, and easily lose transparency. These drawbacks have prevented the materials from finding utility in practical applications. Moreover, a CO.sub.2 gas laser which emits a high-energy beam has a disadvantage that the input and output terminal faces of the fiber are easily broken by the beam to induce the problem of discharge breakdown. To date, the transmission of light of the middle infrared band by the optical fiber has been found infeasible.
Primarily for the purpose of eliminating the restrictions imposed as described above on the conventional optical fiber for the CO.sub.2 gas laser, the inventors have proposed a novel optical waveguide based on an entirely new principle. To facilitate the comprehension of this invention, this optical waveguide will be briefly described below. If a given material has a refractive index n.sub.r smaller than 1 at the working wavenumber of CO.sub.2 laser light of 940 cm.sup.-1 for example, a light obliquely incident upon this material (which is assumed to have an amply smooth surface) can be expected to be totally reflected. If a clad is made of this material with a hollow core embraced therein and a light of a wavenumber of 940 cm.sup.-1 is injected into this hollow-core waveguide, then the optical wave of the light can be guided with amply low loss. The inventors examined numerous materials to find a material possessing such characteristics. They have consequently found that glass formed preponderantly of SiO.sub.2 possesses a refractive index smaller than 1 in the middle infrared band. For the wavenumber of 940 cm.sup.-1 of the beam generated by the CO.sub.2 gas laser, however, the hollow-core waveguide produced by using this material has too large loss for the waveguide to prove thoroughly feasible.
Japanese Patent Application Disclosure No. SHO 51(1976)-9843 discloses a hollow pipe made of quartz in a construction such that a light of high intensity such as a laser beam can be guided thereby when the light is injected at an angle chosen to cause total reflection of the light on the inner wall surface of the pipe. This disclosure, however, makes no specific mention at all of the wavenumber of the light to be guided, the refractive index of the material to be used, the angle of incident light to the hollow pipe, etc.